The invention relates to a mammography accessory for MR elastography (MR=magnetic resonance), as well as to an MR apparatus equipped with a mammography accessory of this kind.
MR elastography utilizes the fact that the phase in an MR image of the object to be examined changes under the influence of mechanical waves acting on the object to be examined. The extent of this change is dependent on the deflection (that is, the shift out of the state of equilibrium) of the tissue that is caused by the mechanical waves. Information concerning given mechanical parameters of the tissue, for example, concerning the elasticity, can thus be derived from the MR phase images, that is, images reproducing the phase of the nuclear magnetization.
Mammography is a preferred field of application of MR elastography, that is, the imaging of the breast (mamma). A mammography accessory which is suitable for this purpose is known from U.S. patent application Ser. No. 09/743,659 in which mechanical waves are excited in the mamma by an oscillation applicator which oscillates horizontally and perpendicularly to the longitudinal direction of the patient to be examined. A drawback in this respect is the long duration of the examination performed by means of such an accessory (up to 30 minutes). An arrangement as disclosed in U.S. Pat. No. 5,952,828 has a similar drawback. The oscillation applicators then act from below on the mammae of the patient in the prone position, thus producing an oscillation tangentially to their contact surface. However, in essence only transverse waves are excited in the mammae; such transverse waves, however, have a penetration depth which is less than that of longitudinal waves.
It is an object of the present invention to provide a mammography accessory which is suitable for reducing the examination time and excites predominantly longitudinal mechanical waves in the mammae.
This object is achieved in accordance with the invention by means of a mammography accessory for elastography which includes a positioning unit which is provided with a supporting surface in which at least one opening is formed so as to receive at least one of the mammae and also includes an oscillation generating arrangement for generating mechanical waves via at least one oscillation applicator which performs a reciprocating motion in the longitudinal direction of a patient to be examined and has a contact surface which extends at least approximately perpendicularly to its oscillation direction.
Like in the known mammography accessories, the mammae are compressed to a given extent in the direction perpendicular to the contact surface of the oscillation applicators. However, whereas in the known accessories the compression causes an increase of the dimensions of the mammae in the longitudinal direction of the patient, the compression of the mammae in accordance with the invention causes a decrease of these dimensions because the contact surface has an effect in this direction. The imaging of the mammae by forming a number of slice images in the direction perpendicular to the longitudinal direction (this orientation of the slice images is necessary to enable simultaneous imaging of the breast wall and the axillary areas) requires a smaller number of slices and hence a shorter period of time for the acquisition of the MR data required for these slices.
A further advantage resides in the fact that the mammae appear in substantially non-distorted form in the individual slice images when this direction of compression is employed. In the case of the known mammography accessories, however, the mammae appear in these slice images so as to be distorted from the side or from below; this impedes the diagnosis and is not acceptable to many examiners.
One embodiment of the invention enables adaptation to different anatomical circumstances. The compression may then be chosen to be such that on the one hand the mamma is fixed but on the other hand it can still be elastically deformed. A further embodiment enables the fixation of the mammae also at the side of the oscillation applicator. The oscillation applicator itself may then be stationary relative to the positioning unit.
The positioning unit may in principle have a comparatively wide opening for receiving both mammae. Another embodiment, however, offers the advantage that each mamma can be enclosed by an MR receiving coil, thus enabling undisturbed reception of the MR signals. Moreover, the adjacently arranged MR receiving coils enable SENSE measurements so that the examination time is reduced even further.
In order to carry out MR elastography the magnetic gradient fields act on the examination zone simultaneously with the mechanical waves. This is achieved by means of an MR apparatus that includes the subject accessory.
In accordance with one aspect of the invention, a mammography accessory for MR elastography is provided. The accessory includes a positioning unit which is provided with a supporting surface in which at least one opening is formed so as to receive at least one of the mammae, and also includes an oscillation generating arrangement for generating mechanical oscillations via at least one oscillation applicator which performs a reciprocating motion in the longitudinal direction of a patient to be examined and has a contact surface which extends at least approximately perpendicularly to its oscillation direction.
In accordance with a more limited aspect of the present invention, the mammography accessory includes a first compression plate which is arranged at the side of the opening which faces the oscillation applicator and is adjustable in the oscillation direction of the oscillation applicator.
In accordance with a more limited aspect of the invention, the mammography accessory includes a second compression plate which is arranged at the side of the opening which faces the first compression plate, which is mounted so as to be stationary relative to the positioning unit and through which the oscillation applicator acts on the zone between the two compression plates.
In accordance with a more limited aspect of the invention, the mammography accessory includes two openings for a respective mamma, at the area of each opening there being provided at least one MR coil which is situated in a plane parallel to the supporting surface and serves to receive MR signals, and also an oscillation applicator which forms part of the oscillation generating arrangement.
In accordance with another aspect of the invention, an MR apparatus is provided. The MR apparatus includes a mammography accessory as described herein and also includes a generator which controls the variation in time of magnetic gradient fields, and a control unit which controls the generator and the mammography accessory in such a manner that the mechanical oscillations generated by the mammography accessory and the magnetic gradient fields are synchronized relative to one another.